1. Field of the Invention
The invention relates to a surgical hood, and more particularly to a patient hood for retaining heat during surgery and recovery and to provide access for visual inspection and treatment.
2. Description of the Related Art
Recent clinical studies have demonstrated that mild perioperative and postoperative hypothermia is associated with several significant adverse effects on patient health. For example, in cases in which the core body temperature of a patient is moderately depressed, surgical wounds have been shown to heal more slowly, and be more susceptible to infection; duration of hospitalization may be two to three days longer, on average; blood loss during surgery is increased; incidences of serious cardiac complications are significantly more common; postanesthetic recovery is longer; and patient comfort during recovery is significantly reduced.
Maintaining a patient's core temperature during surgery can be a significant problem, inasmuch as the patient's metabolism is typically depressed due to anesthesia, and surgical theaters are commonly maintained at a relatively cool temperature, for the comfort of the surgical team. Meanwhile, the patient is commonly lightly draped, with portions of the patient's body exposed to the cool environment. Additionally, in cases where the patient is undergoing major surgery, the patient may lose significant body heat directly from the surgical site. Studies have also shown that the face and upper chest are far more sensitive to temperature loss than other regions of the body.
Body heat is lost by a combination of four sources:
(a) Conduction heat loss is the passage of heat energy from one mass to a cooler mass in direct contact therewith. In the case of a human body it occurs when a portion of the body is in contact with a surface or medium that is colder than that portion of the body. Heat is conducted from the warmer region to the colder region, thus drawing heat away from the body. In a cool environment, air in contact with the body will draw heat from the body.
(b) The second source of thermal energy loss is convection. Convection occurs in a fluid when one portion of the fluid is warmer than another, causing motion in the fluid, as the warmer, less dense, fluid rises, while the colder, denser fluid drops. Convection occurs in conjunction with conduction, when fluid, such as air, is warmed in a localized area by conduction through contact with a warmer mass, and is then carried away from the heat source by convection, which draws cool air into contact with the mass. Convection is also used to describe heat transfer due to motion or circulation of fluid by other sources. Thus, convection heat losses in a body increase when portions of the body are exposed to moving or circulating air that is cooler than the body.
(c) A third source of heat loss is evaporative heat loss. Evaporative heat loss occurs when a fluid in contact with a surface evaporates into the surrounding atmosphere. The energy required for the fluid to transition from a liquid state to a gaseous state is significant, and it is generally drawn from the surface in the form of heat. The human body's normal heat regulatory system exploits this phenomenon by producing sweat. As sweat evaporates from the skin, the skin is cooled, Thus, by conduction the body core temperature is regulated. During the course of a surgical procedure, cleansers, disinfectants, and other fluids are placed in contact with the body, together with the sweat that is normally produced. Heat is thus drawn from the body due to conduction and evaporation.
(d) The fourth source of heat loss is radiation. Radiation, mostly in the infrared spectrum, propagates outward from a warm mass and is a function of the absolute temperature of the mass, regardless of the ambient temperature, and, unlike the other three sources, is independent of any transmission medium. Radiation heat loss occurs when the energy radiated from a body exceeds heat either produced by the body or taken in by the body from other sources, such as conduction and convection.
Of the four sources of heat loss, conduction and convection contribute the most to the loss of core body temperature during surgery. An extensive examination of the causes and effects of heat loss to patients may be found in an article entitled Complications and Treatment of Mild Hypothermia (Anesthesiology, V95, No. 2, August 2001).
Methods and devices for mitigating the loss of heat during and immediately following surgery include the use of thermal blankets of various designs, which are placed around the patient during or following surgery. Also known is the use of heated surfaces on which a patient is placed during surgery. In U.S. Pat. No. 5,877,279, issued to Elting et al. on Mar. 30, 1999, a surgical garment is disclosed in which a patient is draped with a lightweight garment having several components that covers most of the body. The garment includes a hood that is configured to cover the top and the back of the patient's head, while leaving the patient's face exposed. However, this solution is incomplete, inasmuch as a significant percentage of heat loss occurs at the patient's face. This has been recognized, and occasional informal attempts have been made to address this issue. For example, in some cases practitioners have improvised by placing a plastic trash bag over a patient's head and face during surgery to prevent this heat loss.
There are, however, several drawbacks with using plastic bags. The bag obscures the patient's face, which should be visible to the anesthesiologist for the purpose of monitoring the patient's condition. The bag tends to be generally unwieldy and difficult to situate and typically the patient will have an endotrachial tube in the nose or throat, requiring an opening in the bag to permit passage of the tube. It is at least uncomfortable, and possibly dangerous for a patient to wear such a bag while conscious, such as during procedures requiring only local anesthesia or during recovery.
These and other disadvantages, including the difficulty of appropriately configuring such a bag, and the danger to the patient imposed by the obscuring of the patient's face, make the use of a plastic bag less than ideal.